Estrogen therapy is associated with reduced risk of coronary artery disease in postmenopausal women. The atheroprotective effects of estrogen may in part be due to favorable changes in lipids, but other biological effects may also be important. Cytomegalovirus, a potential atherogenic agent, causes rapid increases in intracellular reactive oxygen species which are essential for transcription factor (NFkappaB) activation. NFkappaB in turn activates the immediate early promotor (MIEP), with subsequent expression of the major immediate early protein product IE-72 which trans activates the MIEP, all critical events for viral replication. Because estrogen and the anti-estrogen tamoxifen have antioxygen properties, we examined the effect of estrogen and tamoxifen on generation of the active oxygen species using an intracellular fluorescent marker (dichloroflourescein) and on the early intracellular events following cytomegalovirus infection of human coronary smooth muscle cells. We found that 17beta-estradiol at physiological concentrations (0.1- 0.5 nM) dose-dependently inhibited cytomegalovirus-induced reactive oxygen species accumulation, as did tamoxifen at therapeutic concentrations (5-500 nM); progesterone (2.5-25 nM was ineffective. To determine the relevance of this finding, we next co-transfected smooth muscle cells with a reporter gene under the control of MIEP and an IE-72 expression vector. IE-72 stimulated MIEP transcriptional activity 2-3 fold, in effect inhibited by estrogen and by tamoxifen. Estrogen or tamoxifen, but not progesterone, lowered viral titers and reduced cytopathic effects (plaque formation) by 50% 8 days after cytomegalovirus infection. We conclude that estrogen and tamoxifen have anti-cytomegalovirus effects by inhibiting reactive oxygen species generation following infection of coronary smooth muscle cells. To the extent that cytomegalovirus is a risk factor for atherosclerosis, this antioxidant effect may contribute to the anti-atherogenic effect of hormone therapy.